The present invention relates to a method of and an apparatus for evaluating the reliability of metal interconnects. In particular, the present invention is applicable for a method of and an apparatus for evaluating the reliability of metal interconnects of semiconductor devices.
The evaluation of metal interconnect reliability is important. For example, in the field of semiconductor devices, along with the strong demand toward the high integration of very high LSIs, the evaluation of metal interconnect reliability becomes important. In particular, there is required a reliability evaluating means compatible with the actual device operating environment.
As the method of evaluating the reliability of metal interconnects, there has been used an electromigration evaluating test for a long time. This test is performed by applying a specified current to a metal interconnect at an environment temperature of about 200.degree. C., and then measuring the lifetime either until the breakage occurs or until the resistance increase over a threshold value. On the other hand, in recent years, there have arisen a problem of a failure mode due to the so-called stress-induced migration, that is, damages of metal interconnects due to mechanical factors, for example, the breakage of metal interconnect due to a stress applied from a cover film or the like. A stress-induced migration evaluating test has been used to evaluate the above failure due to the stress-induced migration.
As for the testing time, the electromigration evaluating test takes a time ranging from several minutes to several hundreds hours; while the stress-induced migration evaluating test takes a time of several thousands hour. Accordingly, these failure phenomena are different from each other in the time required for generation of a failure.
In the actual operating environment, electromigration and stress-induced migration seem to progress simultaneously; accordingly, even when these two acceleration tests are separately performed, the actual operating environment cannot be sufficiently reproduced.
For example, in a layered Al interconnect 12 with a barrier metal 11 such as TiN shown in FIG. 4, a slit-like void shown in FIG. 5 is possibly generated as a failure after constant-temperature storage testing. In this case, the prior art electromigration evaluation fails to detect the above failure because the resistance is little changed and the electric conduction is kept by the redundant effect of the barrier metal. Namely, as typically shown in FIG. 6 a current flows in the barrier metal 11 along the path shown by the numeral 14, so that the breakage is evaluated. On the other hand, when the electromigration evaluation is made separately from the above stress-induced evaluation in accordance with the conventional manner, the interconnect with no slit-like void is, of course, evaluated to be excellent, and the interconnect in which slit-like voids 13 are generated to some extent not to cause breakage is also evaluated to be excellent.
However, when slit-like voids are generated, they actually lead to the breakage. As shown in FIG. 7, a slit-like void 13 is inevitably grown along the direction of arrow 15 by Joule heating upon current-carrying, which eventually causes the breakage as shown in FIG. 8. The prior art method cannot detect such a failure as shown in FIG. 5.
Even when the electromigration test without stress-migration test evaluates an interconnect with line width of 0.3 .mu.m or 0.2 .mu.m such that MTF (mean time to failure until 50% of cumulative percent defective) is excellent, the stress-induced migration test possibly evaluates the interconnect to be defective.